French authorities have sent riot police to the southern port city of Marseille after a group of teenagers burned a bus, gravely injuring a young passenger.

President Jacques Chirac issued a statement early Sunday condemning the arson attack as a despicable act. He vowed that everything will be done to find those responsible.

French police were braced for a surge of violence as Friday was the anniversary of the start of three weeks of rioting in poor immigrant suburbs of Paris last year.

Authorities say a group of three or four young people forcibly boarded the Marseille bus late Saturday, doused it with a flammable liquid and set it on fire before fleeing.

Authorities say the 26-year-old female passenger who was severely burned is breathing on a respirator today. Three other passengers were treated for smoke inhalation. All city bus service has been suspended.

Another bus was torched by two assailants late Friday in the northern Paris suburb of Clichy-sous-Bois where last year's rioting first erupted. No injuries were reported in that attack.

Rioting last year was fueled by immigrant anger at what they see as France's failure to provide equal opportunities for its five million Muslims and other ethnic minorities.